FIG. 1 shows the waveform of an idealized data signal waveform. In this case, binary “1”s are represented by a particular relatively high voltage level and binary “0”s by a particular relatively low voltage level. However, when such signals are transmitted through real data transmission lines, the waveform can become filtered; in particular at high data rates the capacitance of the transmission lines can be significant and acts to give the transmission lines a low pass characteristic. Such a situation arises, for example, in the data transmission lines of the backplane of a multi-card electronic system.
FIG. 2 shows a data waveform like that of FIG. 1 after it has been passed through a transmission line of significant capacitance. (There are approximately 50 bits in the time frame of the figure.) As can be seen, there is a difficulty in sampling the data because the sections of the waveform representing “1”s are not always above the threshold level and the sections representing “0”s are not always below the threshold level, the threshold level being that used to discriminate between “1”s and “0”s, which in this case is zero volts. This means that “1”s can be incorrectly sampled as “0”s and vice versa.
A known solution to this problem is to pass the data waveform through an equalizing filter before it is sampled. The equalizing filter is given an inverse filter characteristic to that of the transmission line, thus compensating for it and restoring the waveform to something near ideal (i.e. like FIG. 1), which can then be cleanly sampled. A problem with this approach is that the amount of equalization required can vary from installation to installation of even nominally the same equipment and can vary with time, even in the same installation. If insufficient equalization, or indeed too much equalization, is provided, then data sampling errors can still occur.
A known equalizing filter employed to solve this problem is shown in FIG. 3. Here the data waveform received from a transmission line 1 is applied in parallel to a set of band pass filters 2, which have neighbouring pass bands. The outputs of the filters are recombined with a summing amplifier 3. Each filter 2 has its gain set by an automatic gain control so that each band contains the same signal power.
A problem with the circuit of FIG. 3 is that the assumption that each band has the same signal power is incorrect. The signal spectrum is not always flat and, of course, the spectrum also depends on the channel coding scheme used and can vary from time to time with the content of the data stream.
The invention therefore provides a different kind of equalization that attempts to avoid this problem and which has other advantages, as will become apparent.